The present invention relates to a method for making natural oil-based polyols. Still further, the present invention includes using natural oil-based polyols to produce polyurethane resins for use as casting compounds for electrical applications.
Polyols may be produced from petroleum. However, there has been an active trend in recent years to use renewable resources, such as vegetable and animal oils. Vegetable and animal oil molecules must be chemically transformed in order to introduce hydroxyl groups. For instance, soybean oil does not contain any hydroxyl groups but has on average about 4.6 double bonds per molecule. The unsaturated portions of the vegetable or animal oil molecule can be converted to hydroxyl groups. However, many reactions for preparing polyols from various natural oils are not very selective. By-products, in addition to alcohol groups, are created during the transformation. Furthermore, many conventional methods of preparing polyols from natural oils do not produce polyols having a significant content of hydroxyl groups. Still further, many available methods of preparing polyols from natural oils do not produce products having a desirable viscosity. Greases or waxes often result as a consequence of such chemical transformations.
Conventionally, cast electrical components such as dry voltage transformers and insulators are formed from epoxy resins. Epoxy resins are rather expensive to use. Still further, epoxy resins are not easy to handle at low temperatures and have poor elasticity. Polyurethane resins prepared with castor oil have also been produced. However, these resins tend to be rubbery and thus undesirable for certain casting applications. Still further, castor oil-based polyurethanes have some limitations due to their higher price and environmental problems related to their by-products.
In order to overcome the deficiencies found with conventional processes for making natural oil-based polyols, a method for making natural oil-based polyols from vegetable or animal oil or epoxidized vegetable or animal oil is needed for a variety of applications including preparation of, through polyurethane chemistry, a resin for use as an electroinsulating casting compound, which is another embodiment of the present invention. Still further, this method of making natural oil-based polyols should avoid substantial side reactions, such as esterification, cyclization, polymerization, crosslinking, and other undesirable reactions, and should produce polyols having a high hydroxyl content and a desirable viscosity.
According to the present invention, a method for making natural oil-based polyols directly from vegetable or animal oil using a consecutive two-step process involving epoxidation and hydroxylation is provided. Specifically, this process comprises adding a peroxyacid to a natural oil wherein said natural oil and said peroxyacid react to form an epoxidized natural oil and adding said epoxidized natural oil to a mixture of an alcohol, water, and a fluoboric acid catalyst. The catalytic amount of fluoboric acid is less than about 0.5% by weight of the entire reaction mixture, and the amount of water is about 10 to 30% by weight of the entire mixture. The epoxidized natural oil undergoes hydroxylation and forms a natural oil-based polyol. The present invention further includes a method for making natural oil-based polyols from epoxidized oil by hydroylating the epoxidized oil in the presence of fluoboric acid, alcohol and water in the amounts discussed above.
The natural oil-based polyols created by this method may be reacted with isocyanates so as to form polyurethanes, which is another embodiment of the present invention. Alternatively, fillers such as silica may be combined with these natural oil-based polyols before they are reacted with isocyanates to form polyurethanes. In still another embodiment of the present invention, polyurethanes made from natural oil-based polyols may be used to form electroinsulating casting resins for use in electrical applications.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The method of the present invention involves making natural oil-based polyols by converting each of the double bonds of the natural oil molecule into a hydroxyl group. This method takes place at approximately atmospheric pressure.
The process of the present invention involves epoxidation and subsequent hydroxylation of a natural oil so as to make a polyol. More specifically, the process of the present invention includes adding a peroxyacid in a solvent to a vegetable or animal oil wherein the oil and the peroxyacid react to form an epoxidized oil, and adding said epoxidized oil, which is in the solvent, to a mixture of an alcohol, water, and a catalytic amount of fluoboric acid so as to form a natural oil-based polyol. These are consecutive, non-stop steps. The reaction is not stopped after the epoxidized natural oil forms so as to purify the intermediate product.
Another embodiment of the present invention is a method of making polyols from epoxidized natural oil by the second step of the process outlined above. This second step provides a fast conversion of an epoxidized oil to a polyol. Epoxidized natural oils, the starting component of this second step, can be obtained from Ferro Corporation, C. P. Hall, Ashland Chemicals or Union Carbide or made from natural oil, as discussed above. Preferably, the epoxidized natural oil used in the method of the present invention has an epoxide content between 6-7% by mole epoxy groups per mole of epoxidized natural oil, and it has about 90 to 95% of the double bonds in the natural oil epoxidized. In other terms, preferably, each epoxidized natural oil molecule has about 2-6 epoxy groups. In still other terms, preferably, the epoxidized natural oil used has an epoxide content of 2-8 weight %.
Any vegetable or animal oil may be used in this process. Examples of vegetable and animal oils that may be used include, but are not limited to, soybean oil, safflower oil, linseed oil, corn oil, sunflower oil, olive oil, canola oil, sesame oil, cottonseed oil, palm oil, rapeseed oil, tung oil, fish oil, or a blend of any of these oils. Alternatively, any partially hydrogenated or epoxidized natural oil or genetically modified natural oil can be used to obtain the desired hydroxyl content. Examples of such oils include, but are not limited to, high oleic safflower oil, high oleic soybean oil, high oleic peanut oil, high oleic sunflower oil (NuSun sunflower oil) and high erucic rapeseed oil (Crumbe oil). The iodine values of these natural oils range from about 40 to 220 and more preferably from about 80 to 180. When natural oils having lower iodine values are used to make natural oil-based polyols, polyols with lower hydroxyl numbers and thus lower viscosities are created.
Any peroxyacid may be used in the epoxidation reaction. Examples of peroxyacids that may be used include, but are not limited to, peroxyformic acid, peroxyacetic acid, trifluoroperoxyacetic acid, benzyloxyperoxyfornic acid, 3,5-dinitroperoxybenzoic acid, m-chloroperoxybenzoic acid, or any combination of these peroxyacids. The peroxyacids may be formed in-situ by reacting a hydroperoxide with the corresponding acid, such as formic or acetic acid. Examples of hydroperoxides that may be used include, but are not limited to, hydrogen peroxide, tert-butylhydroperoxide, triphenylsilylhydroperoxide, cumylhydroperoxide, or any combination of these hydroperoxides. Preferably, the peroxyacid is in a solvent such as acetic acid, formic acid, or chloroform.
Fluoboric acid is used as the acid catalyst in the hydroxylation step. Using fluoboric acid as a catalyst in this hydroxylation reaction works better than using other inorganic acids suggested by the prior art. Specifically, by using fluoboric acid, the reaction time is shorter, the reactivity is higher, and the natural oil-based polyols produced consistently have a higher hydroxyl content. A catalytic amount of fluoboric acid is used in this reaction. This amount should be below about 0.5% by weight of the entire reaction mixture or about 1.25% by weight of the amount of epoxidized oil used. Usually, the amount of fluoboric acid is between about 0.1% and 0.5% by weight of the total reaction mixture, and preferably, it is between about 0.3% and 0.5% by weight of the entire reaction mixture.
Examples of alcohols or alcohol mixtures that may be used in the hydroxylation reaction include, but are not limited to, monoalcohols such as methanol, ethanol, propanol, isopropanol and butanol. It is desirable to use at least some methanol in the hydroxylation reaction because it is the most reactive alcohol. Methanol may be used with solvents other than alcohols, such as chloroform, toluene, formic acid, or acetic acid. It is important during the hydroxylation step to always have an excess amount of alcohol present so as to prevent polymerization and the formation of products having higher viscosities.
Water is also an important component in this reaction. It reacts with the epoxy groups of the epoxidized natural oils to form two hydroxyl groups per epoxy group in some locations so as to increase the hydroxyl content of the natural oil-based polyols. Specifically, water contributes about 10% or lower to dihydroxylation of the natural oil. Still further, it acts as a diluent to the fluoboric acid so that the acid is not reactive towards undesired cleavage of the triglyceride linkages present in the natural oil molecules. About 10 wt % to 30 wt % water should be used in this reaction. In the method of the present invention, the OH content of the polyol can be more precisely controlled than previous methods by varying the amount of water used in the reaction.
The mixture of alcohol and water is crucial in this reaction. The ratio of alcohol to water defines the polarity of the reaction media and limits the solubility of the epoxidized natural oil and the polyol in the reaction mixture. One phase is an alcohol/water mixture with a dissolved part of epoxidized natural oil and a dissolved part of newly formed polyol. The rest of the epoxidized natural oil is separated as undissolved droplets, the same as the undissolved part of the newly formed polyol. The reaction effectively takes place between dissolved epoxidized natural oil and alcohol/water. The reaction also takes place on the surface of the epoxidized natural oil droplets. In both cases, the molar ratio of alcohol/water to epoxy groups is many times higher than the stoichiometric ratio of alcohol/water to epoxidized natural oil, which strongly promotes the main reaction between alcohol and epoxy groups and diminishes undesirable side-reactions.
The molar ratio of alcohol to water should be between about 2:1 and 9:1. Preferably, the ratio of alcohol to water is between about 3:1 and 5:1. Preferably, this mixture is a water/methanol, water/isopropanol, or water/ethanol mixture. Most preferably, it is a water/methanol mixture. When methanol is the alcohol used, the polyol obtained has the best balance between OH number and viscosity. Polyols with higher OH numbers can be created with isopropanol/water mixtures, but these polyols also have higher viscosities. The hydroxylation reaction takes place at approximately the boiling temperature of the water/alcohol mixture.
Another benefit of the reaction taking place in a heterogeneous reaction system is the lower solubility of the polyol in the alcohol/water mixture. According to data in Table 1, shown below, the solubility of the newly-formed polyol decreases strongly with increasing water content in the mixture. In fact, when the amount of water present is between 10-30 wt %, the solubility of the polyol is decreased from 100% to 7.5%, which decreases the possibility of reaction between the epoxidized natural oil and the newly-formed polyol and suppresses the formation of high molecular weight molecules.
Water also reacts with epoxy groups giving vicinal diols and increasing the OH number of the polyol. By regulating the alcohol to water ratio, it is possible to regulate the OH number of the polyol and find the best balance between the desirable OH number and viscosity. However, larger amounts of water decrease the reaction rate and thus require a larger amount of catalyst. A higher catalyst concentration promotes undesirable side-reactions such as hydrolysis, transesterification and crosslinking giving products of higher viscosity and lower OH number. Therefore, the alcohol to water ratio should be in certain limits, namely between 2:1 and 9: 1, giving the preferable concentration of the dissolved epoxidized natural oil in the reaction mixture. At that concentration of dissolved epoxidized natural oil, the conversion of epoxy groups is the fastest, and undesirable reactions are minimal. Accordingly, the OH number of the polyol is the highest. Table 1 shows the effect of methanol/water ratio on the solubility of epoxidized soybean oil and soy polyols at the boiling temperature of the methanol (64xc2x0 C.) and on the OH number and viscosity of the synthesized polyols.
Subsequent to the hydroxylation reaction, a neutralizing agent should be added to neutralize the fluoboric acid catalyst. Neutralizing agents that may be used include, but are not limited to, weak bases, metal bicarbonates, or ion-exchange resins. Preferably, the neutralizing agents are weakly-basic anionic ion-exchange resins. Examples of suitable neutralizing agents include, but are not limited to, ammonia, ammonia/water solution, calcium carbonate, sodium bicarbonate, magnesium carbonate, amines, and/or anionic ion-exchange resins.
The polyols made by the method of the present invention have a viscosity in the range of 1.0-7.0 Paxc2x7s at room temperature. The viscosity of these polyols is lower than polyols made by other methods because the method of the present invention avoids substantial side-reactions, such as polymerization or crosslinking. Still further, the natural oil-based polyols made by the method of the present invention have a hydroxyl content ranging from 110 to 213 mg KOH/g. Preferably, the polyol has a high hydroxyl content which equals approximately one hydroxyl group per double bond of the natural oil. Natural oil-based polyols can be made in yields of 85-95% using any of the various embodiments of the process of the present invention.
By making polyols in a boiling reaction mixture, epoxidized natural oil is converted to polyols very quickly. The method of the present invention gives a low level of side reactions such as polymerization, cyclization, esterification, and crosslinking so as to produce polyols of narrow Molecular Weight Distribution (MWD). In fact, transesterification and diglyceride formation is negligible in this process.